fix checking issue with most pieces set to priority 0

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9302 a83610d8-ad2a-0410-a6ab-fc0612d85776
minor invariant check tweak

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9304 a83610d8-ad2a-0410-a6ab-fc0612d85776
fix build with encryption disabled

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9306 a83610d8-ad2a-0410-a6ab-fc0612d85776
update and regenerate reference documentation

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9308 a83610d8-ad2a-0410-a6ab-fc0612d85776